Have an insurance backup plan to be covered knowing you gave notice legally.
It's easy by letter by recorded delivery and put your account number and name etc in the letter. make sure it gets to them before the date on your price increase and notification of change of T&Cs email/letter. State you are using the Ofcom clause to exit penalty free and refer to the statement about leaving in the email/letter sent by VM. Not a formal letter like a job application so don't worry.
See
https://www.virginmedia.com/help/leaving
Virgin Media, Sunderland, SR43 4AA
If they don't answer or disconnect you, the letter is legal evidence as it's sent recorded to their address they state.